Apparatus for providing a steep voltage step across a load in electric high-voltage circuit

ABSTRACT

An apparatus for providing a steep voltage step across a load has two sets of electric conductors with electrodes forming spark gaps, said sets of conductors being connected to be charged in parallel by high-voltage source and discharged in series by spark gaps in order to impress a multiple of the tension of said highvoltage source upon the load and said sets of conductors being shaped and arranged to form around such of said spark gaps a transmission conducting zone in which the impedance continuously varies in a direction away from the spark gap and is at its minimum at the outer boundary of the zone in order to make the voltage step steep by impedance transformer action for the high frequencies generated at breakthrough in the spark gap.

United States Patent APPARATUS FOR PROVIDING A STEEP VOLTAGE STEP ACROSSA LOAD IN ELECTRIC HIGH- VOLTAGE CIRCUIT 4 Claims, 4 Drawing Figs.

US. Cl 307/106 Int. Cl ll03k 3/00 Field of Search 250/93;

References Cited Primary Examiner-Robert K. Schaefer AssistantExaminer-H. J. Hohauser Att0rneyJohn Lezdey ABSTRACT: An apparatus forproviding a steep voltage step across a load has two sets of electricconductors with electrodes forming spark gaps, said sets of conductorsbeing connected to be charged in parallel by high-voltage source anddischarged in series by spark gaps in order to impress a multiple of thetension of said high-voltage source upon the load and said sets ofconductors being shaped and arranged to form around such of said sparkgaps a transmission conducting zone in which the impedance continuouslyvaries in a direction away from the spark gap and is at its minimum atthe outer boundary of the zone in order to make the voltage step steepby impedance transformer action for the high frequencies generated atbreakthrough in the spark gap.

APPARATUS FOR PROVIDING A STEEP VOLTAGE STEP ACROSS A LOAD IN ELECTRICHIGH-VOLTAGE CIRCUIT In high voltage technics it is often desired tohave the possibility of providing extremely steep voltage steps across aload. One example is the X-ray flash technique, and for the sake ofsimplicity the following description will treat of this technique,though the present invention has many other uses.

In the X-ray flash technique there is used as energy source a pulsegenerator which can deliver a high instantaneous effect in the dischargetube. It is desired, int. al. that the generator pulse has as short arise time as possible and a well defined pulse length and pulseamplitude, and that the generator has a low output impedance. It wastried up to now to satisfy these requirements substantially by one oftwo expedients. One expedient implies the use as energy storing unit ofa coaxial cable which is charged by means of a high-voltage source anddischarged by a gas discharge, for example in a high-pressure spark gap.When high initial voltages are required this method is complicated asone has to use a complicated high-voltage source, such as a Van deGraaff generator. The other expedient implies the use of so-called Marxgenerators which as energy storing unit utilize a number of capacitorswhich are charged in parallel connection by means of parallel connectionresistances and/or inductances, but are discharged in series throughspark gaps. Pulse-producing networks of capacitors and inductances maypossibly be substituted for the capacitors. The problem associated withthese devices when used together with, e.g., X-ray flash tubes, is int.al. difficulty of providing sufficiently low-inductive systems andsufficiently steep voltage steps.

This invention has for its object to overcome the drawbacks associatedwith the prior art devices of the type above accounted for. Theinvention thus relates to an apparatus for providing a steep voltagestep across a load in an electric highvoltage circuit which comprises atleast one spark gap formed by two electrode means and connected to ahigh-voltage source and to the load, said spark gap being adapted atbreakthroughs to generate the voltage step across the load.Characteristic of this apparatus is that the electrode means areconnected to or constitute part of two relatively electrically insulatedelectric conductor means which together form a transmission conductingzone which surrounds the spark gap and in which the impedancecontinuously varies, preferably monotonously sinks, in a direction awayfrom the spark gap and is at its minimum at the outer boundary of thezone, said zone serving as energy storing capacitor along or togetherwith further capacitor means connected to the conductor means. Theapparatus according to theinvention provides steeper voltage stepsacross the load than do the prior art pulse generators, and permitsgeneration of very short duration pulses whose voltage amplitude canreadily be varied. The apparatus according to the invention also is of asimple construction allowing large pulse generators to be built by theassembly of mutually identical units.

The invention will be more fully described in the following, referencebeing had to the accompanying drawings which illustrate someembodiments.

in the drawings:

FIG. 1 is a diagrammatic view of a simple apparatus according to theinvention serving to elucidate the principles underlying the invention;

FIG. 2 is a diagrammatic view of a Marx type pulse generator realized byapplication of the present invention;

FIGS. 3, and

F IG. 4 show structural details in a pulse generator according to FIG.2.

F16. 1 shows two electrode means I and 2 in the form of two facing,central, elevated portions on two circular coaxial plates 3 and 4 whichare planar disregarding the electrode elevated portions which areinsignificant both in point of height and diameter. The plates with theelectrode means thereon are relatively electrically insulated, and asuitable dielectric, such as a gas or a gas mixture, is provided betweenthem. The electrode means 1 has a central hole into which one end of atrigger electrode 5 penetrates in an electrically insulated manner. Atrigger current source 8 is connected via lines 6 and 7 to the (DCelectrode 5 and the plate 3. A high-voltage source (DC voltage) 9 isconnected by means of the line 7 to the plate 3 and by means of a line10 to one end of a load 11, such as an X-ray flash tube, and aresistance or inductance 12 connected parallel with the load. The otherend of the load 11 and the component 12 is connected to the plate 4 bymeans of a line 13.

The plates 3 and 4 are charged to a high-voltage difference by means ofthe high-voltage source via lines 7, l0 and 13 and the resistance orinductance l2, whereupon the trigger current source 8 is caused togenerate a breakthrough in the trigger spark gap between the electrode 1and the trigger electrode 5, a breakthrough that initiates abreakthrough between the electrode means 1 and 2, the energy stored inthe plates 3 and 4 being rapidly discharged through load 11 (littleenergy has time to escape through the resistance or inductance 12) inthe form of a substantially rectangular pulse of extremely small risetime.

The electrode means I and 2 form a spark gap for positively bringingabout a discharge of the plates 3 and 4 at their center axis, and theplates form a transmission conducting zone which surrounds the spark gapbetween the electrode means I and 2 and in which electric energy isstored in order then to be discharged by a spark at the center of thezone. Such a transmission conducting zone formed by circular planarplates 3 and 4 has int. al. the following properties:

I. The wave velocity of the electromagnetic waves which at the dischargeat the center of the plates move in an outward direction towards theperiphery of the plates and in an inward direction towards the centeraxis thereof, is dependent upon the distance of the waves from thecenter axis of the plates.

2. The impedance in the transmission conducting zone at any point of thezone is proportional to the distance between the plates 3 and 4 andinversely proportional to the distance of the point from the center axisof the plates.

lt follows from the said two properties that the transmission conductingzone will function as an impedance transformer for high frequencies. Atthe discharge in the center the highest frequencies will be amplifiedand give rise to extremely steep voltage and current steps across and inthe load, respectively. As a result, the discharge at a suitable loadassumes the shape of a rectangular pulse of very small rise time and arelatively small fall time which is dependent upon the load. The energysupplied to the load will thus become greater than at an ordinarysinusoidal discharge pulse. Since the impedance varies with the distancebetween the plates 3 and 4, more rapid and steeper pulses can beobtained by reducing said distance.

The use of a gas or a gas mixture between the plates 3 and 4 and theirspark gap electrode means 1 and 2 has many advantages of which int. al.the following may be mentioned. The breakthrough voltage is readilyvaried by variation of the gas pressure. The gas is regenerated aftereach discharge and need not thus be renewed between discharges. Themechanical construction is the same for different gases. However, thereis nothing to prevent the use of liquids or solid insulators between theplates and the spark gap electrodes. If the spark is allowed to traversefluid or solid substances the length of the spark passage for a givenvoltage applied can be considerably reduced, which will alsoconsiderably improve the pulse properties. When particularly steepvoltage steps are desired the use of a plastics foil of goodbreakthrough resistance is recommended as material between the plates.In such a case, of course, the construction must be so adapted that thefoil is readily exchangeable between discharges.

The pulse duration and the energy stored can be varied by variation ofthe dielectricity constant for the dielectric between the plates 3 and4, or by changing the dimensions of the plates. It is also possible toconnect special energy storing capacitors to the plates forming thetransmission conducting zone.

In the above reasoning the discussion was conducted starting from thefact that the plates 3 and 4 forming the transmission conducting zonesurrounding the spark gap are planar and parallel. The same reasoning,however, applies even if the plates are of another shape, e.g., conicalwith facing apices. What is essential is that the conduct meanscollectively constituting the transmission conducting zone surroundingthe spark gap are so designed and arranged that the impedance in thetransmission conducting zone continuously varies, preferablymonotonously sinks, in a direction away from the spark gap and is at itsminimum at the outer boundary of the zone. Thus in a direction away fromthe spark gap there must not occur in the transmission conducting zoneany such discontinuous impedance changes as would imply reflexion ofelectromagnetic waves moving through the zone in a direction towards oraway from the central spark gap.

When higher voltages are desired a plurality of transmission conductingzone units according to FIG. 1 can be built together to form a Marx-typepulse generator. An example of this is diagrammatically shown in FIG. 2.The generator according to FIG. 2 is enclosed is a pressure vessel 14having two manhole covers 15 which in a manner not shown serve asfastenings for gas and electricity lead-in bushings and as fasteningsfor a load 11 built into the vessel, such as an X-ray flash tube (or fora leadout bushing for a cable drawn to an outer load). Erected on thebottom of the vessel are electricity insulating supports 16 which carryan electrically insulating plate 17. Placed on the plate 17 is anelectrically conductive capacitor plate 18 and above said plate thereare carried on electrically insulating supports 19 upstanding from theplate 17, several, in the present instance three, pairs of plates 3 and4 forming transmission conducting zones and associated spark gap-formingelectrode means 1 and 2. These plates 3 and 4 with the associatedelectrode means 1 and 2 are of the construction shown in FIG. 1 exceptthat the uppermost plate 3 only is provided with a trigger electrode 5,while the other plates 3 have an electrode means 1 without any hole fora trigger electrode, and except that the uppermost plate 3 is extendedpast the supports 19 and is electrically connected to the wall of thepressure vessel. Extending between the uppermost plate 3 and the bottomof the vessel is an electrically insulating cylindrical wall 20separating the cylindrical sidewall of the vessel and the supports 16,19 to increase the electric breakthrough resistance of the space betweenthe sidewall of the vessel and the electrically conductive means carriedby the supports beneath the uppermost plate 3. In the case selected,pressure gas is used in the vessel as a dielectric in the space 21between the two plates 3 and 4 of the same pair, which form atransmission conducting zone between them. The plates 4 and 3 followingupon each other and belonging to different pairs constitute energystoring capacitors, and use is preferably made between them of a solidor fluid substance as a dielectric 22. The same applies for thecapacitor formed by the lowermost plate 4 and the plate 18.

The plates 4 are series connected in sequence by means of chargingresistances or inductances 23, and the uppermost plate is connected bymeans of a similar resistance or inductance 23 to a conductor 24 whichis connected toone terminal of a source of high direct voltage, providedoutside the vessel. The other terminal of the DC voltage source isconnected via a line 25 to the uppermost plate 3 and as a consequence tothe electrically conductive wall of the vessel 14, to which also oneterminal of the X-ray flash tube 11 is connected. The capacitor plate 18and the plates 3 are series connected in sequence over chargingresistances or inductances 26, and the plate 18 besides is connected viaa central line 27 to the other terminal of the X-ray flash tube 11. Thetrigger electrode 5 and the uppermost plate 3 are connected to an outertrigger circuit via lines 28,29.

The relatively slow charging of the alternating transmission conductingzones and capacitor zones between the plates 3, 4 and 18 by means of thehigh-voltage source takes place in parallel connection over theresistances or inductances 23, 26

in such a way that the plates 3 and 18 assume the potential for oneterminal of the high-voltage source and the plates 4 assume thepotential for the other terminal of the high-voltage source, while therapid discharge by ignition of the spark gaps between the electrodes 1and 2 in the transmission conducting zones with the aid of the triggerelectrode 5 takes place according to the Marx generator principle inseries connection of the energy storing units so that the pulse throughthe load 11 will have a considerably higher voltage amplitude than whatcorresponds to the output voltage of the charging highvoltage source.

The setting of the spark gaps between the electrodes 1 and 2 in thegenerator is such that the spark gaps as far as possible have the samebreakthrough voltage. Practically, this is realized in that the sparkingdistance is made equally long in all gaps. This will also make itpossible, when a gas or gas mixture is used as a dielectric in the sparkgaps, to readily vary the breakthrough properties by variation of thegas pressure in the vessel.

Of essential importance for the function of the generator as an electricenergy source of short rise time is that the ignition of the spark gap1, 2 having the trigger electrode 5 rapidly results in ignition of alsothe other spark gaps l, 2. The construction according to FIG. 2satisfies this requirement. During breakthrough in the spark gap 1, 2having the trigger electrode 5 there occur in the associatedtransmission conducting zone 21 rapid variations of the potentialdistribution on the conductor means 3 and 4 of this zone. The strongcapacitive connection between said spark gap and the following sparkgaps 1, 2 via the intermediate capacitor zone results in that thevariations of potential distribution in the spark gap 1, 2 having thetrigger electrode are transmitted also to the following spark gaps l, 2.This will also force said latter spark gaps to be ignited. Thiscircumstance plays a decisive role for the design of the ignition sparkgaps in those cases when two or more such gaps are incorporated with theconstruction, and for the formation of a modulus system so that thegenerator can be built together by series connection of several unitseach comprising a number of transmission conducting zones having sparkgaps and alternating capacitor zones.

FIGS. 3 and 4 show suitable detail embodiments of a generator accordingto FIG. 2.

In FIG. 3, the plate 18 is shown attached to the conductor 27 (connectedto the load) by means of a screw 28 whose head fits in the outward bulgeof the adjacent plate 4 constituting the electrode means 2. Between theplates 18 and 4, in the energy storing capacitor zone formed by them,there is a solid dielectric 29, and the peripheral portions of theplates are embedded in a distance ring 30 of electrically insulatingmaterial, such as epoxy resin. Correspondingly, the supetjacent plates3, 4 constituting a capacitor zone between them are embedded in pairs attheir peripheries in electrically insulating distance rings 30 and havebetween them a solid dielectric 29. When the electrode means 1, 2 areconstituted by bulges on the plates 3 and 4 proper instead of by meansmounted thereon, the dielectric 29 should be provided with electricallyconductive foils 31 within the area of the bulges, said foils being inelectric contact with the respective adjacent plate 3, 4. To ensure aconstant distance between the plates 3, 4 defining transmissionconducting zones, in the case of large diameter plates, electricallyinsulating distance rings 32 are provided on said plates also near thecenters thereof. With the use of a gas or a gas mixture as a dielectricin the transmission conducting zones the distance rings 30, 32 areprovided with gas penetration holes (not shown). To the outer distancerings 30 are connected two contact strips 33 to each of the plates 3, 4and 4, 18, respectively, in the ring. The two contact strips in eachring are diametrically opposed and encased in insulating material 34.These contact strips are offset peripherally from distance ring todistance ring so that only two of the contact strips are visible in FIG.3. These contact strips serve for the connection of the chargingimpedances (details 23 and 26 in FIG. 2). Through the contact stripsbeing offset as described,

the charging impedances will have an extension substantially parallelwith the planes of the plates 3, 4, I8 and a length suitable withrespect to the charging voltage. To prevent unfavorable effect of dust,if any, on the plates 3, 4, for instance in the form of unintentionalbreakthroughs in the transmission conducting zone, a disk 35 ofinsulating material can be inserted in the transmission conducting zonesbetween the distance rings 30, 32, as has been shown for the sake ofsimplicity only for one of the transmission conducting zones in FIG. 3.By varying the thickness of the disk 35 also the length of the sparkgap, that is the distance between the electrodes 1 and 2, can be finelyadjusted.

In FIG. 3, the dielectric 29 in the capacitor zones and the insulatingdisk 35 have been shown as homogeneous layers. Same as at themanufacture of ordinary capacitors it is often advantageous to useinstead of a single layer, several layers for increasing thebreakthrough resistance. At voltages over about 50 kv. it is alsopossible to insert metallic layers or sheets between the insulatinglayers in the capacitor zones and in the transmission conducting zones.In the latter case a good optical and ionizing contact must be ensuredbetween the different dielectric layer through holes at the centers ofthe metal sheets.

FIG. 4 shows a suitable embodiment when several trigger spark gaps arerequired for instance for meeting requirements for insignificant timejitter of the discharge. The uppermost plate 3 has attached to it aspecific electrode means la which has a central hole for the triggerelectrode 5. The uppermost plate 4 is designed in the manner describedin connection with FIG. 3 and thus has a bulge acting s an electrodemeans 2, and it is separated from the subjacent plate 3a by means of aninsulating layer 29, as already described. The plate 3a has no centralbulge and carries instead at its lower side a metallic distance device36 which has attached to it at the underside a metal plate 37 having acentral hole for accommodating a spark gap electrode means 38 projectingfrom the underside of the plate 37. said electrode means 38 has acentral hole into which penetrates a trigger electrode 39 which extendsthrough a passage which leads through the distance device 36 and isfilled with insulating material 40. The trigger electrode 39 and thedistance device 36 are connected by mean of a coaxial cable 41 whichextends through the field-free space between the plates 3a and 37 out ofthe generator between the distance rings 30, to an outer trigger circuit(not shown) to produce a triggering spark between the trigger electrode39 and the electrode 38 simultaneously as a triggering spark is producedbetween the trigger electrode 5 and the electrode la.All the plates 3and 4 situated beneath the plate 37 are designed in the manner describedin connection with FIG. 3. Should further trigger spark gaps benecessary they can be given the construction described in connectionwith the plates 3a and 37.

The construction of trigger spark gaps described in connection with theplates 3a and 37 provides a good capacitive connection between thevarious stages despite the arrangement of an additional trigger sparkgap. Of course, it must be seen to it at the construction of the triggercircuits that breakthroughs in the various trigger spark gapsare'obtained as simultaneously as possible and that the trigger circuitsfor the additional trigger spark gaps are given a design that permitsthe main electrode associated with the trigger spark gap, e.g.,electrode 38 in FIG. 4, to assume a high potential in the course of thedischarge.

What I claim and desired to secure by Letters Patent is:

I. An apparatus for providing a steep voltage step across a load in anelectric high-voltage circuit, comprising a high-voltage source, aplurality of pairs of electrode means, each pair forming a spark gap,two sets of electric conductor means, means electrically insulating oneset of said conductor means for the other set, said conductor meansbeing arranged in an array with alternate conductor means belonging tothe same set, impedance means DC connecting said connector means of eachset separately, means connecting said conductor means to said voltagesource and the load, at least some of said conductor means beingprovided with said electrode means and being shaped and arranged to formaround each pair of said electrode means and the spark gap formedthereby a transmission conducting zone which surrounds the spark gap andin which the impedance continuously varies in a direction away from thespark gap and is at its minimum at the outer boundary of the zone.

2. An apparatus according to claim 1 in which said conductor meansprovided with said electrode means are shaped and arranged to formaround each pair of electrode means a transmission conducting zone inwhich the impedance monotonously sinks in a direction away from thespark gap.

3. An apparatus for providing a steep voltage step across a load in anelectric high-voltage circuit, comprising a high-voltage source, aplurality of pairs of electrode means, each pair forming a spark gap,two sets of circular electric conductor plates, means electricallyinsulating one set of said plates from the other set, said plates beingarranged coaxially in an array with alternate plates belonging to thesame set, impedance means DC connecting said plates of each setseparately, means connecting said plates to said voltage source and theload, at least some of said plates being provided at their center axiswith said electrode means and being shaped and arranged to form aroundeach pair of said electrode a. and the spark gap formed thereby atransmission conducting zone which surrounds the spark gap and in whichthe impedance continuously sinks in a direction away from the spark gapand the center axis of said plates.

4. An apparatus according to claim 3, in which said plates aresubstantially planar and parallel.

1. An apparatus for providing a steep voltage step across a load in anelectric high-voltage circuit, comprising a high-voltage source, aplurality of pairs of electrode means, each pair forming a spark gap,two sets of electric conductor means, means electrically insulating oneset of said conductor means for the other set, said conductor meansbeing arranged in an array with alternate conductor means belonging tothe same set, impedance means DC connecting said connector means of eachset separately, means connecting said conductor means to said voltagesource and the load, at least some of said conductor means beingprovided with said electrode means and being shaped and arranged to formaround each pair of said electrode means and the spark gap formedthereby a transmission conducting zone which surrounds the spark gap andin which the impedance continuously varies in a direction away from thespark gap and is at its minimum at the outer boundary of the zone.
 2. Anapparatus according to claim 1 in which said conductor means providedwith said electrode means are shaped and arranged to form around eachpair of electrode means a transmission conducting zone in which theimpedance monotonously sinks in a direction away from the spark gap. 3.An apparatus for providing a steep voltage step across a load in anelectric high-voltage circuit, comprising a high-voltage source, aplurality of pairs of electrode means, each pair forming a spark gap,two sets of circular electric conductor plates, means electricallyinsulating one set of said plates from the other set, said plates beingarranged coaxially in an array with alternate plates belonging to thesame set, impedance means DC connecting said plates of each setseparately, means connecting said plates to said voltage source and theload, at least some of said plates being provided at their center axiswith said electrode means and being shaped and arranged to form aroundeach pair of said electrode a. and the spark gap formed thereby atransmission conducting zone which surrounds the spark gap and in whichthe impedance continuously sinks in a direction away from the spark gapand the center axis of said plates.
 4. An apparatus according to claim3, in which said plates are substantially planar and parallel.